In a mobile communications network, when carrier signals of a plurality of frequencies pass through some passive components, a passive intermodulation (PIM) signal is generated. So-called passive intermodulation means that a plurality of carrier signals of different frequencies are intermodulated due to a frequency mixing effect when they pass through a passive component of a system, and as a result, additional interference is caused to the system. For a mobile communications system, passive components include a duplexer, a feeder cable, a tower-mounted amplifier, an arrester, an antenna, and the like. Because a passive component has an unreliable mechanical connection and a stained contact surface, uses a material with a hysteresis characteristic, and the like, non-linear frequency mixing occurs at a material joint for signals of different frequencies, and PIM interference signals of different amplitudes are generated. A PIM interference signal within a receive frequency band of a base station receiver is received by the base station receiver and causes interference to a received signal of the receiver. As a result, a signal-to-noise ratio of the received signal is decreased, thereby reducing a capacity of the communications system and decreasing network quality. Therefore, a PIM product needs to be suppressed or cancelled.
To reduce impact of the PIM product on the received signal, a PIM cancellation technology is usually used in a baseband unit (BBU), in a remote radio unit (RRU), or in a transit device between a BBU and an RRU, to improve quality of the received signal.
In an existing PIM cancellation technology, during PIM cancellation, non-linear modeling is performed on a transmit signal and a PIM interference signal to obtain a cancellation signal used to cancel the PIM interference signal, and the cancellation signal is superposed on a receive channel of a BBU in a reverse-phase manner, to cancel PIM generated by the transmit signal.
However, when PIM cancellation is performed in a BBU, a cancellation signal is obtained by performing modeling based on a baseband signal to be sent by the BBU to an RRU, and the signal is further processed by functional units such as a multi-carrier combiner, a crest factor reduction (CFR) unit, a digital pre-distortion (DPD) unit, and a duplexer in the RRU. As a result, a transmit signal finally output by the RRU cannot be precisely restored on a BBU side, and modeling precision is low. The cancellation signal obtained after modeling performed by the BBU cannot completely cancel an actual PIM interference signal. As a result, interference still exists in a received signal on an uplink. In addition, in a multi-carrier scenario, implementation is complex and many resources are consumed.
When PIM cancellation is performed in an RRU, each RRU can obtain only a PIM interference signal generated by a transmit signal on a transmit channel of the RRU. Therefore, PIM cancellation between a plurality of RRUs cannot be implemented.
When PIM cancellation is performed in a transit device between a BBU and an RRU, if two different RRUs connected to a same BBU are provided by different device manufacturers, interfaces between the BBU and the RRUs are different; to be specific, transit devices are different. Therefore, PIM cancellation cannot be performed in the transit devices. In addition, in a multi-carrier scenario, implementation is complex and many resources are consumed.
In conclusion, in a wireless communications system, a problem that PIM interference affects receiving performance of a communications device exists, and problems exist in the existing PIM cancellation technology, for example, the modeling precision is low, many resources are consumed, and an application scenario is limited.